The present invention relates to thermal ink-transfer printers and, particularly to thermal ink-transfer printers which are capable of printing on paper surfaces longer than the circumference of the outer surface of the platen roller without being limited by the circumference of the outer surface of the roller.
Conventional thermal ink-transfer printers roughly comprise, as shown in FIG. 1, a platen roller 1 which has printing paper 2 supplied to its outer surface and which winds the paper 2 around the outer surface by counterclockwise rotation, a clamper 3 for pressing the paper 2 against the outer surface of the roller 1, an ink film 4 which is moved in direction A of FIG. 1 in accordance with the rotation of the roller 1 and to which is applied a fusible or sublimational ink on a surface thereof, and a thermal head 5 which presses the ink film 4 against the paper 2 and transfers ink applied to the ink film 4, to the paper 2 by means of heat of a heating element 6, on the basis of signals supplied correspondingly with the rotation of the platen roller 1.
The conventional printer as mentioned above, in the case of printing a frame of color picture or character, uses ink film 4 having a color frame which comprises a plurality of primary color patterns such as yellow, deep blue (cyan blue), and magenta, as well as black to a corresponding one frame of a picture. Since the printing paper 2 is conveyed with the rotating platen roller 1 being positioned and held by the clamp 3 and since each line to be printed is printed progressively in accordance with the state of rotational phase of the platen roller 1, it is quite difficult to cause an accurate registration made sequentially by these color patterns provided on the ink film 4 for each frame of a picture.
However, conventional printers also have the following problems:
(a) The length of the printing paper 2 is restricted to within the outer surface length of the platen roller 1;
(b) The enlargement of the diameter of the platen roller 1, in order to make it possible to print large pictures, causes the picture quality to degrade due to an inherent decrease of the pressure of the heat element 6 attached to the thermal head 5. The larger the diameter of the platen roller 1, the broader the total contact area to the head assembly. As the pressure per unit area of the contact portion decreases, if pressure to the head assembly is fixed, this phenomenon reduces the relative pressure between the head elements arranged in a line-shape and both the ink film 4 and paper 2 on the platen roller 1, so that a satisfactory print cannot be obtained unless pressure force increases;
(c) The enlargement of the platen roller 1 requires an increase in the pressure of the thermal head 5 as described in the above item (b), and the consequent enlargement with increased rigidity which is needed to cause the printer to become heavy and bulky and to increase the manufacturing cost; and
(d) The increase of the pressure of the thermal head 5 to match the enlargement of the roller 1 causes the driving mechanism of the roller 1 to become large, making it difficult to control the roller 1.
In order to eliminate some of the defects described in the above items (a) to (d), the alternative configuration shown in FIG. 2 is conventionally provided as another example of FIG. 1. The printer in FIG. 2 roughly comprises a platen roller 1 for winding a printing paper 2A longer than the length of the circumference of the outer surface of the roller, a pinch roller 7 for pinching the paper 2A between the roller 1 in order to transport the paper 2A by-directionally. Other elements with the same numbers as FIG. 1 are either identical or equivalent. The printer shown in this example is capable of continuously printing on paper 2A which is longer than the length of the circumference of the outer surface of the roller 1. The roller 1 makes a clockwise rotation to bring the paper 2A back to a starting position after each printing pass. The thermal head 5 presses the ink film 4 to the paper 2A during a feed of the paper 2A together with the ink film 4 for each printing pass paper 2. Printing to the paper 2A is performed by repeating this process for each color pattern.
Though the defects in the items (a) to (d) are eliminated by means of the printer shown in FIG. 2, there is the following further problem.
In the case of printing a color picture to the paper 2A, the paper 2A has to be returned to the starting position of printing whenever each color pattern of the color frame is printed. While the paper 2A advances and returns, the paper 2A is apt to displace with respect to the platen roller 1, therefore it is a problem of off-registration in the colors printed on the paper 2A. Accordingly, this type of conventional printer is unsuitable for printing color pictures of high definition image.